Vibrations are becoming increasingly significant for the design and configuration of motor vehicles and internal combustion engines. It is attempted, inter alia, to influence and model specifically the sound generated by the internal combustion engine. Measures in this context are also combined under the term sound design. Such development work is also motivated by the realization that the customer's decision to purchase a motor vehicle is influenced to a significant and increasing degree, even decisively, by the sound of the internal combustion engine or of the vehicle. For example, the driver of a sports car prefers a vehicle or engine whose sound emphasizes the sporty character of the vehicle.
In the scope of a sound design, vibrations are compensated, e.g., eliminated or extinguished, or individual vibrations of a specific frequency are isolated, filtered out and, if appropriate, modeled.
The sources that may be differentiated as noise sources on a motor vehicle include flow noise, noise due to the emission of solid-borne sound, and noise due to the introduction of solid-borne sound into the vehicle bodywork via the engine mount.
Flow noise includes, for example, the noise at the mouth of the exhaust, the intake noise and the noise of the fan, while the noise due to the emission of solid-borne sound includes the actual engine noise and the emission of the exhaust system. The engine structure which is made to oscillate by shocks and alternating forces irradiates the solid-borne sound as air-borne sound via its engine surfaces and in this way generates the actual engine noise.
The introduction of solid-borne sound via the engine mount, in particular the introduction of solid-borne sound into the vehicle bodywork, is of particular significance for the acoustic driving comfort.
The internal combustion engine and the associated secondary assemblies are systems which are capable of oscillating and whose oscillatory behavior can be influenced. The most relevant components with shock excitation and force excitation are the crank casing, the cylinder block, the cylinder head, the crank drive, the piston and the valve drive. These components are subject to the mass forces and gas forces. The crank drive comprises here, in particular, the crankshaft, the piston, the piston pin, and the connecting rod, and forms the system which is capable of oscillating and which is relevant for the method according to the disclosure.
The crankshaft is made to undergo rotational oscillations by the rotational forces which change over time and which are applied to the crankshaft via the connecting rods which are coupled to the individual crank pins. These rotational oscillations give rise both to noise due to the emission of solid-borne sound and to noise due to the introduction of solid-borne sound into the bodywork and into the internal combustion engine. When the crankshaft is excited in the natural frequency range, large rotational oscillation amplitudes may occur which can even lead to fatigue fracture. This shows that the oscillations are of interest not only in conjunction with a sound design but also with respect to the strength of the components.
The rotational oscillations of the crankshaft are transmitted in an undesirable fashion to the camshaft via the control drive or camshaft drive, wherein the camshaft itself also presents a system capable of oscillation and can cause other systems, in particular the valve drive, to oscillate. In addition, the oscillations of the crankshaft are introduced into the drive train, via which they can be passed to the tires of a vehicle.
The rotational force profile at a crankshaft throw of a four-stroke internal combustion engine is periodic, wherein the period extends over two revolutions of the crankshaft. The rotational force profile is normally decomposed into its harmonic components by means of Fourier analysis in order to be able to make statements about the excitation of rotational oscillations. In this context, the actual rotational force profile is composed of a constant rotational force and a multiplicity of harmonically changing rotational forces which have different rotational force amplitudes and frequencies or oscillation rates. The ratio of the oscillation rate n, of each harmonic to the rotational speed n of the crankshaft or of the engine is referred to as the order i of the harmonic.
Due to the high dynamic load on the crankshaft as a result of the mass forces and gas forces, typical internal combustion engines may be designed to implement mass balancing which is as wide ranging as possible, e.g., is optimized. In this context, the term “mass balancing” combines all the measures which compensate or reduce the effect of the mass forces toward the outside. To this extent, balancing the mass forces relates not only to the mass forces as such but also to the moments which are caused by the mass forces.
In this context, an approach to the solution in the targeted adjustment of the throw of the crankshaft, of the number and of the arrangement of the cylinders and of the ignition sequence exists in such a way that the best possible mass balancing is achieved.
A six-cylinder in-line engine can be completely balanced in this way. The six cylinders are combined in pairs in such a way that they run in parallel mechanically as a cylinder pair. The first and sixth cylinders, the second and fifth cylinders and the third and fourth cylinders are therefore combined to form a cylinder pair, wherein the crankshaft pins or crankshaft throws of the three cylinder pairs are each arranged offset by 120° CA on the crankshaft. Running mechanically in parallel means that the two pistons of the two cylinders which run mechanically in parallel are located at the same ° CA (degrees crank angle) at the top dead center (TDC) or bottom dead center (BDC). When a suitable ignition sequence is selected, the mass forces are completely balanced.
In the case of a three-cylinder in-line engine, the mass forces of the first order and the mass forces of the second order can also be completely balanced by selecting a suitable crankshaft throw and likewise a suitable ignition sequence, but not the moments which are caused by the mass forces.
Complete mass balancing, as in the case of the six-cylinder in-line engine described above, cannot always be implemented, and therefore further measures have to be taken, for example arranging counterweights on the crankshaft and/or equipping the internal combustion engine with at least one balancing shaft.
The starting point of these measures is that the crankshaft is loaded by the rotational forces which change over time and which are composed of the gas forces and mass forces of the crank drive. The masses of the crank drive, e.g., the individual masses of the connecting rods, of the piston, of the piston pin and of the piston rings, can be transferred into an oscillating equivalent mass and a rotating equivalent mass. The mass force of the rotating equivalent mass can easily be balanced in terms of their external effect by counterweights arranged on the crankshaft.
The balancing of the rotating mass force caused by the oscillating equivalent mass is more complex, said mass force being approximately composed of a mass force of the first order, which rotates at the engine speed and a mass of the second order which rotates at twice the engine speed, with higher order forces being negligible.
The rotating mass forces of any order can be virtually balanced by the arrangement of two shafts, referred to as balancing shafts, which rotate in opposite directions and are provided with corresponding weights. The shafts for the balancing of the mass forces of the first order rotate here at the engine speed and the shafts for balancing the mass forces of the second order rotate at twice the engine speed. This type of mass balancing is very costly, complex and has a high spatial requirement.
In addition, even in the case of complete balancing of the rotating mass forces, mass moments arise since the mass forces of the individual cylinders act at the central planes of the cylinders. These mass moments can be compensated in turn in an individual case by a balancing shaft which is equipped with weights. The latter increases the spatial requirement, the costs and the weight of the entire mass balancing, and therefore those of the drive unit additionally.
The moments caused by the mass forces of the first order, for example in the case of a three-cylinder in-line engine, can be compensated by a single balancing shaft which rotates at the engine speed in the opposite direction to the crankshaft and at whose ends two balancing weights which are arranged offset by 180° and serve as an unbalance are provided.
The provision of one balancing shaft or, if appropriate, a plurality of balancing shafts not only increases the spatial requirement and the costs, but also the fuel consumption. The increased fuel consumption is caused, on the one hand, by the additional weight of the balancing unit, in particular of the shafts, and of the counterweights which serve as an unbalance and which perceptively increase the overall weight of the drive unit. On the other hand, the balancing unit with its rotating shafts and other moving components contributes significantly to the frictional loss of the internal combustion engine and to the increase of this frictional loss. The latter has a relevance, in particular, due to the fact that the balancing unit is always and continuously operational as soon as the internal combustion engine starts and is operated. The mass forces are balanced continuously here without it being taken into account whether or not the instantaneous operating state of the internal combustion engine at all demands such mass balancing, for example for reasons of the sound design.
It would therefore be possible to dispense with balancing of the moments caused by the mass forces of the first order in a three-cylinder in-line engine at relatively high engine speeds since the noise caused by the oscillations are evaluated as being problematic only at low rotational speeds and during idling. On the other hand, at relatively high rotational speeds there is no indication for mass balancing for reasons of the sound design.
The conventional balancing units are also disadvantageous in that only a small degree of freedom of maneuver is present in terms of the structural configuration and the arrangement in the engine compartment. The balancing shafts which are usually used are driven mechanically on the crankshaft side via a belt drive or a pair of gearwheels and are therefore generally arranged underneath the crank casing.
The inventors herein have recognized the issues with the above approaches and offer an approach for mass balancing which includes a small space requirement, a small frictional loss and more structural freedom of maneuver in terms of the configuration of the balancing. Accordingly, a method for balancing mass forces of a crank drive of an internal combustion engine having at least one cylinder comprises providing at least one balancing unit which has at least one balancing weight which serves as an unbalance and which rotates about a rotational axis when the balancing unit is operational, the at least one balancing unit being embodied as a switchable balancing unit, and switching on the at least one balancing unit as a function of at least one operating parameter of the internal combustion engine.
The method according to the disclosure for mass balancing makes use of a switchable balancing unit which is activated, e.g., switched on, when indicated, but which can be deactivated, e.g., switched off, when not indicated. Examples of a switchable balancing unit are an electrically operated balancing unit in which, for the purpose of deactivation, the power supply is disabled, or a mechanically driven balancing unit in which switching off is carried out by interrupting the drive, for example by providing a clutch which, in the open position, interrupts the force flux from the belt drive or gearwheel drive to the at least one balancing shaft.
The balancing unit is switched here as a function of at least one operating parameter of the internal combustion engine. In this way, the method according to the disclosure permits mass balancing in a three-cylinder in-line engine at low rotational speeds and during idling, with mass balancing being dispensed with at relatively high rotational speeds by switching off the balancing unit, in order to reduce the frictional loss and therefore the fuel consumption. The use of a switchable balancing unit therefore permits mass balancing which is distinguished by a relatively low frictional loss.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.